It is known to provide a motor vehicle having a four wheel drive mode of operation in which motive power is supplied to each of two pairs of wheels of the vehicle. Motive power is supplied to the wheels by means of a driveline.
Some known vehicles are arranged such that motive power is permanently supplied to both pairs of wheels. Some other vehicles are arranged such that motive power is selectively supplied to either only one pair or to both pairs of wheels. A driver operable selector may be provided to allow the driver to select two wheel or four wheel operation. Some driveline systems require the vehicle to be stationary when transitioning between two and four wheel drive modes. Such systems may be referred to as static disconnect/reconnect systems.
GB2407804 discloses a dynamic driveline reconnect arrangement in which reconnection of two of the wheels to the driveline following disconnection of the wheels from the driveline may be undertaken when the vehicle is moving. Such a system may be referred to as a dynamic driveline reconnect system. The system disclosed in GB2407804 employs clutch arrangements to enable dynamic driveline reconnection.
In some known dynamic driveline reconnect systems the vehicle is operable automatically to disconnect the driveline to two of the wheels when a prescribed condition is met so that the vehicle operates in a two wheel drive mode. The system automatically reconnects the driveline to enable four wheel drive operation when the prescribed condition is not met.
It is known to provide a front wheel drive vehicle having a dynamic driveline that includes a power transfer unit (PTU) that includes a clutch device having a synchroniser. The PTU is configured to connect a prop shaft of the vehicle to a transmission of the vehicle in order to drive rear wheels of the vehicle. The PTU can connect and disconnect the prop shaft from the transmission whilst the vehicle is moving.
It is to be understood that reconnect operations are known to be performed where the releasable torque transmitting device includes a clutch device having a synchroniser. The clutch device may for example be a dog-clutch device or a single or multi-plate clutch device. The synchroniser device is configured to synchronise a speed of rotation of the input and output portions of the clutch device prior to closure of the clutch device, and may occasionally fail correctly to close, resulting in slippage between the input and output portions of the clutch device. If a vehicle accelerates relatively harshly, front wheels of the vehicle may suffer spin that is significantly greater than that of rear wheels of the vehicle. One way to remedy the situation is to perform a reconnect operation. Accordingly, in some known vehicles a reconnect operation may be triggered in dependence on the speed of a first set of wheels with respect to a second set, for example when a speed of front wheels of the vehicle exceeds that of rear wheels of the vehicle by more than a prescribed amount. In some known vehicles the reconnect operation may be repeated several times in rapid succession in order to attempt to ensure that the clutch device closes correctly.
It is an aim of embodiments of the present invention to at least partially mitigate the disadvantages of known dynamic driveline systems.